This invention relates to a surface acoustic wave (SAW) filter having interdigital transducers (IDTs) and reflectors formed on a piezoelectric substrate.
SAW filters are coming to be used in communication equipment of different types for providing compact apparatus. For use in a portable telephone, however, filters having a wide passband of over 25 MHz and a low level of insertion loss are required. Examples of prior art SAW filter satisfying such requirements, and in particular the requirement of a wide passband, include those using a 36.degree. rotation Y cut X propagation (Y-X) LiTaO.sub.3 substrate or a 64.degree. Y-X LiNbO.sub.3 substrate (referred to, for example, in U.S. Pat. No. 5,300,902) with a large electro-mechanical coefficient.
Among SAW filters using a 36.degree. Y-X LiTaO.sub.3 substrate, designs such as the ladder type and the interdigitated interdigital transducer (IIDT) type are actually being used. With SAW filters of an ordinary type using mode coupling, however, a band width of only about 2% of the frequency at the center can be obtained. Since a ratio of at least 3% is required between the band width and the center frequency for a portable telephone, these filters are not useful for practical application.
SAW filters using a 64.degree. Y-X LiNbO.sub.3 substrate, on the other band, have been in use for portable telephones because a ratio of about 4% is obtainable with them between the band width and the center frequency. Neither are these filters practical, however, because they are not easy to manufacture and their production cost is high.
Explained more in detail, SAW filters used in a portable telephone have a stop band at least on one side of its passband, that is, at least either on its low frequency side or on its high frequency side, and a certain level of attenuation must be provided within such a stop band or bands (usually over 20 dB). In other words, filters which have a good shape factor (defined as the ratio between the passband width and the attenuation band width) and are easy to manufacture are those having a narrow band width at attenuation of 20 dB.
FIG. 8 is a frequency characteristic of a prior art SAW filter using a 64.degree. Y-X LiNbO.sub.3 substrate designed for use in a mobile communication system requiring a SAW filter with central frequency f.sub.o of 947.5 MHz on the side of the reception circuit for its portable unit. In order to prevent interference from waves of reception frequencies within 45 MHz of the central frequency f.sub.o on its lower frequency side, this filter should preferably have attenuation of over 20 dB within a stop band of 902.5.+-.12.5 MHz (hereinafter referred to as the lower-frequency side stop band C). For image suppression on the higher frequency side, furthermore, this filter should preferably also have attenuation of over 20 dB within another stop band of 992.5.+-.12.5 MHz (hereinafter referred to as the higher-frequency side stop band C).
Since the central frequencies of SAW filters are not exactly the same but fluctuate due to variations in their production conditions, their attenuation characteristics also vary from one product to another. In order to maintain attenuation levels greater than 20 dB on both the lower and higher frequency sides, therefore, a control with correspondingly high level of accuracy is required regarding frequency.
Although it may be ideal to have the central frequency of a SAW filter approximately in the middle between its lower-frequency and higher-frequency side stop bands C and C', as shown in FIG. 8, in order to obtain high attenuation on both the lower and high frequency sides, attenuation over 20 dB can be obtained even when changes at the central frequency may be big if the band width A at attenuation level of 20 dB is narrow. In other words, the band width at attenuation of 20 dB plays an important role in obtaining attenuation of over 20 dB on both the lower-frequency and high-frequency sides of the central frequency.
In the example shown in FIG. 8, the allowance for production errors (width of allowed frequency variations) on the lower-frequency side is represented by the frequency difference between Points X and Y, and its magnitude is obtained by subtracting the half width at attenuation level of 20 dB (28 MHz) and the half width of the lower-frequency side stop band C (12.5 MHz) from the frequency difference B (=45 MHz) between the centers of the stop band and the passband. In the example shown in FIG. 8, this is calculated to be 4.5 MHz but, since the frequencies of SAW filters normally fluctuate by about .+-.3 MHz from normal-temperature value as temperature changes within a range of, say, -25 to 75.degree. C., this is usually subtracted from the calculated value and 4.5-3=1.5 MHz is regarded as the allowance for production errors on the lower-frequency side. The production error allowance on the higher-frequency side is similarly calculated to be 1.5 MHz.
Since the central frequency of a filter depends heavily on the thickness of its film electrodes and the width of its electrode lines, however, a production error allowance of 1.5 MHz on each side is not sufficient, and it becomes necessary to strictly control the thickness of the film electrodes and the width of the electrode lines for each production lot. This makes the production difficult and has the adverse effect of increasing the production cost.